Method of making a sheet-metal part

ABSTRACT

The invention relates to a method for producing components from sheet metal, wherein at least one patch sheet ( 2 ) is placed on a base sheet ( 1 ) to form a material doubling and is positionally securely fixed, the component unit thus formed, comprising base sheet ( 1 ) and patch sheet ( 2 ), is heated to a temperature suitable for hot forming and is then hot-formed to form the component and, preferably subsequently or simultaneously, is subjected to partial or total cooling or quenching for the purpose of a specific structure conversion, wherein, for the purpose of improved corrosion protection, the base of sheet ( 1 ) in the contact region of the patch sheet ( 2 ), or the patch sheet ( 2 ) on the side thereof facing the base sheet ( 1 ), or both base sheet ( 1 ) and patch sheet ( 2 ) in the corresponding area are coated with an anti-corrosion coating ( 6 ) that is resistant to the temperatures occurring during the hot forming, before the patch sheet ( 2 ) is placed on the base sheet ( 1 ).

The invention relates to a method of making sheet-metal parts, particularly from sheet steel, wherein at least one patch sheet is placed on a base sheet to double thickness of the material and is fixed thereto against relative movement, the subassembly of base sheet and patch sheet formed in this manner is heated to a temperature that is suitable for hot working, and is then hot-worked to form the part and, preferably subsequently or simultaneously, subjected to partial or total cooling or quenching for the purpose of targeted structural transformation.

Parts used in motor-vehicle bodies of all kinds are increasingly being created by methods in which the part is heated to a high temperature and then hot-worked and cooled and hardened in a targeted manner.

With such processes, parts of the highest strength and sufficient flexibility are obtained.

The raw material used is often protected against corrosion by a zinc coating, but increasingly also by a coating made of an aluminum/silicon compound, although this provides significantly poorer protection from corrosion.

In order to meet the requirements for anticorrosion protection, such parts have hitherto been coated with anticorrosion lacquers in later production phases, predominantly by cathodic dip coating (CDC). Here, paint is electrostatically or electrophoretically deposited on the product.

If surfaces of parts to be made are covered by other parts during the hot-working process or later joining processes, this results in doubling the thickness of the material, for example for reinforcement purposes. However, the coating is not able to penetrate and be deposited in the joint between the base sheet and the patch sheet applied during subsequent cathodic dip coating. The joints between the parts are therefore not only incompletely protected against corrosion, but they will also tend to fill with liquids, especially through capillary action. This means that corrosion takes place at a substantially accelerated rate between the parts.

Taking this prior art as a point of departure, it is the object of the invention to provide a method of this generic type with which improved anticorrosion protection is achieved between the juxtaposed faces of the base and patch sheets.

To achieve this object, the invention proposes that, before placement of the patch sheet onto the base sheet, the base sheet (where it contacts the patch sheet) or the patch sheet (on its side facing toward the base sheet) or both the base and patch sheet (in the corresponding region) be coated with an anticorrosion coating that is able to withstand the temperatures of hot working and can also serve to seal this joint and thereby prevent penetration of corrosion-promoting substances.

According to the invention, at least the region between the base and patch sheets is coated with an anticorrosion coating that can withstand the high temperatures that prevail during hot working. The coating takes place before the sheets are placed on top of one another and fixed securely together. The fixation is effected only afterward, with the subassembly consisting of the base and patch sheets that is formed in this manner being heated to a temperature that is suitable for hot working and then hot-worked.

In particular, the corrosion coating is baked in, as it were, during hot working and is thus able to provide durable protection from corrosion.

A provision is preferably made that one of the following paints is used as the anticorrosion coating:

silicon carbide with a suitable solvent, or

polysiloxanes or silanes with a suitable solvent.

The coatings can be easily applied to the surfaces of the corresponding parts in a suitable solvent composition.

A provision is preferably made here that the solvent is

xylenes,

benzenes,

alcohols, or

tetrahydrofuran.

For example, benzene can be used as the benzene, butanol can be used as the alcohol, and solvents with a high vapor pressure, such as tetrahydrofuran, can also be used to increase the viscosity shortly after application. When polysiloxanes or silanes are used, disiloxane, trisiloxane, octamethyltrisiloxane, or decamethylcyclopentasiloxane are preferably used. Coarbosilanes are also suitable as base here.

In addition, a provision is preferably made that a base sheet and patch sheet made of 22MnB5 are used.

A provision is also preferably made that galvanized steel sheet or steel with a zinc-nickel coating or with an aluminum-silicon coating is used as the starting material for the base sheet and the patch sheet.

In addition, a provision is preferably made that the part produced by the method is coated with an anticorrosion paint.

Accordingly, the product of the method is coated with an anticorrosion paint after hot working. However, this need not penetrate into areas between the base sheet and the patch sheet because this area is protected by the anticorrosion coating according to the invention; instead, only the rest of the part is coated with an anticorrosion paint. A provision is preferably made that the part is coated with an E-coat, i.e. an electrophoretically deposited coating, as an anticorrosion paint.

In one especially preferred embodiment, the anticorrosion coating is annealed as a result of the temperature that is applied to the subassembly during hot working.

The anticorrosion coating that is applied between the sheet metal parts is annealed during hot working. No additional energy input is required for this, meaning that the energy that is applied for hot working is sufficient to effect the heat treatment.

In addition, a provision is preferably made that the patch sheet or patch sheets is/are fixed to the base sheet by a joining process.

In some circumstances, a provision can preferably be made that the joining process is a clinching in which the materials of the patch sheet and base sheet are interconnected by local deformation.

A provision can also be preferably made that the joining process is carried out by weld points in one or more areas of the subassembly.

A provision is then preferably made that the weld points are subsequently treated by a laser beam in such a way that they are invisible or hardly visible on the finished subassembly, the treatment being preferably performed after hot working.

The invention further relates to a part that is made by the method according to any one of the claims, the object of the invention being particularly a part that is a structural body part for a vehicle.

A method product according to the invention is illustrated in the attached drawing and described briefly below.

In the drawing:

FIG. 1 is an elevational view of a subassembly of base sheet and patch sheet; and

FIG. 2 is a sectional view of same.

A base sheet 1 and a patch sheet 2 attached thereto can be seen in FIG. 1, a joint 3 being formed between the two sheets. FIG. 2 is a sectional view for clarification, illustrating an end product that was made by the method according to the invention.

A respective patch sheet 2 of the same material quality has been placed onto the base sheet 1, which is for example of a material grade 22 MnB5. The base sheet 1 is provided on its upper face and lower face with an aluminum/silicon layer 4. The patch sheet 2 is also provided on its upper face and lower face with such an aluminum/silicon coating 4.

A coating 6 that is stable at high temperatures and annealed during the hot working of the entire part as a result of the temperatures used during that process is applied in the joint 5 between the two parts 1, 2.

As a result, the joint 5 between the parts 1, 2 is protected against corrosion.

The entire part is coated with an E-coat 7.

The sectional view in particular is shown only schematically. However, the basic structure and the advantages of the method used according to the invention are visible there.

The invention is not limited to this embodiment, but rather can be varied in many respects within the framework of the disclosure. All of the novel individual and combined features disclosed in the description and/or drawing are regarded as being essential to the invention. 

1. A method of making a sheet-metal part from sheet steel, the method comprising the steps of: coating at least one face of the base sheet and/or at least one face of the patch sheet with an anticorrosion paint able to withstand predetermined temperatures at which the steel of the sheets is hot workable; fixing the coated patch sheet on the coated base sheet with the faces juxtaposed at a joint to form a subassembly of double thickness; heating the subassembly of the base sheet and the patch sheet to the predetermined temperatures of hot working; hot-working the heated subassembly to form the part; and cooling or quenching the part for targeted structural transformation, whereby the coating protects the joint and thereby prevents penetration of corrosion-promoting substances into the joint of the part.
 2. The method according to claim 1, wherein one of the following paints is used as the corrosion paint: silicon carbide with a suitable solvent, polysiloxanes or silanes with a suitable solvent.
 3. The method according to claim 2, wherein xylenes, benzenes, alcohols, or tetrahydrofuran is used as solvent in the anticorrosion paint.
 4. The method according to any one of claim 1, wherein the base sheet and the patch sheet are of 22MnB5 grade steel.
 5. The method according to any claim 1, wherein galvanized steel sheet or sheet steel with a zinc-nickel coating or with an aluminum-silicon coating is used as a starting material for the base sheet and the patch sheet.
 6. The method according to claim 1, further comprising the step of: coating the part with another anticorrosion paint after cooling or quenching.
 7. The method according to claim 6, wherein the part is coated with an electrophoretically deposited coating as the other anticorrosion paint.
 8. The method according to claim 1, wherein the anticorrosion paint is annealed by the predetermined temperatures applied to the subassembly for hot working.
 9. The method according to claim 1, wherein the patch sheet is fixed to the base sheet by a joining process.
 10. The method according to claim 9, wherein the joining process is a clinching in which the materials of the patch sheet and base sheet are interconnected by local deformation.
 11. The method according to claim 9, wherein the joining process is carried out by weld points in one or more areas of the subassembly.
 12. The method according to claim 11, wherein the weld points are subsequently treated by a laser beam after hot working in such a way that they are invisible on the subassembly after quenching or cooling.
 13. A part made according to the method according to claim
 1. 14. The part according to claim 13, wherein the part is a structural body part for a vehicle. 